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This is a reland of 858e896d

Original change's description:
> [Audio Focus] Restrict browser group to CrOS
>
> Restrict browser audio focus grouping to Chrome OS.
>
> BUG=915501
>
> Change-Id: I702952cca7a388e78c201531866307797d25b277
> Reviewed-on: https://chromium-review.googlesource.com/c/1379918
> Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
> Commit-Queue: Becca Hughes <beccahughes@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#617800}

Bug: 915501
Change-Id: Ideb9814a6d4ff7e9faedc66efa6d2073adec818d
Reviewed-on: https://chromium-review.googlesource.com/c/1386107Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619747}